In electric resistance heating, the material being heated is heated by joule heat produced by electric current generated in the metal material that is the material being heated and the resistivity of the material being heated. As such, electric resistance heating is viewed as a clean heating method and used particularly in the steel industry but also widely throughout the industrial sector. Electric resistance heating includes one method in which an alternating magnetic flux generated from an electromagnetic coil by passing alternating exciting current through the electromagnetic coil is applied to the material being heated to produce induced current in the material being heated (here called the induction heating method) and another method in which contacts (two electrodes) are brought in contact with the material being heated to directly apply electric current (here called the direct electrical heating method).
As indicated by Equation (1), the frequency of the alternating magnetic flux generated from the electromagnetic coil in the induction heating method or of the electric current directly applied in the direct electrical heating method determines the depth from the surface of the material being heated to which the current passes through the material being heated (penetration depth: δ). Therefore, in order to heat the material being heated to the desired temperature distribution, it is necessary to set the frequency of the current at an appropriate value in view of the diameter, thickness and other aspects of the shape of the material being heated and its electromagnetic property value. Of particular note, is that steel pipe material of ordinary ferromagnetic material has a relative permeability μγ considerably larger than 1 (e.g., 10 to 1000) so that the penetration depth 6 strongly depends on frequency change.δ∝{ρ/(μγ·ƒ)}1/2  <1>where ρ: resistivity of the heated material, relative permeability of the heated material, and f: frequency of the alternating magnetic flux or directly applied electric current.
In this connection, when the electromagnetic coil for induction heating is excited with a heating power supply, the usual practice is to configure a resonant circuit of a capacitor (capacitance: C) connected as disposed in parallel or series with the electromagnetic coil and to apply current at a frequency near the resonant frequency (f) expressed by Equation <2> (see, for example, Japanese Patent Publication (A) 2004-127854 and Japanese Patent Publication (A) H03-1478).f=1/{2π(L·C)}1/2}  <2>,where L means the electromagnetic property value of the heated material, which in the case of the induction heating method is the inductance of the coil system determined by aspects of the coil configuration such as the number of coil winds and dimensions of the electromagnetic coil and the positional arrangement of the electromagnetic coil and material being heated.
Moreover, in order to excite the resonant circuit with good energy efficiency, an impedance matcher for power factor improvement is sometimes installed between the resonant circuit and the heating power supply (see, for example, Japanese Patent Publication (A) 2004-127854, Japanese Patent Publication (A) H03-1478, and Japanese Patent Publication (A) H06-124775).
Patent Publication (A) H03-1478 and Patent Publication (A) H06-124775 teach techniques for performing heating by a method of in advance determining and fixing an appropriate frequency for the thickness, width, steel type and other property and shape aspects of the material to be heated. Patent Publication (A) H03-1478 teaches a high-frequency induction heating device for local annealing of steel pipe and the like, which is an inverter type power supply that enables the frequency of the exciting current to be preset for the electromagnetic coil shape and the like and is not damaged during overload. Patent Publication (A) H06-124775 teaches an inverter-type high-frequency induction heating device for pre-heating or post-heating in butt-welding of steel pipe material and the like, in which multiple electromagnetic coils are provided at the weld zone and the multiple electromagnetic coils are switched to efficiently pass high-frequency current.
In a production line for steel pipe or the like, in order to achieve uniform quality as regards the strength and other material properties of the weld zone and its vicinity, the heating and continuous welding of the weld zone of the heated material while the steel pipe material or other heated material is being conveyed through an induction heating device using an electromagnetic coil or a direct electrical heating device using contacts must be conducted so as to impart the desired shapes/values to the temperature distribution in the direction of weld zone thickness, the shape of the molten weld zone, and the weld frequency fluctuation during welding.
However, during inductance heating of the material to be heated, the inductance L changes greatly with variation in the shape and material properties of the material being heated. Further, differences in the shape of the heated material change the way the heating current flows, which greatly changes not only the heat generation rate and distribution of the heat at the weld zone but also the resulting thickness-direction temperature distribution of the weld zone, shape of the molten weld zone, and weld frequency fluctuation during welding.